Method for coating columbium and alloys thereof



' I 2,991,197 7 METHOD FOR COATING COLUIVIBIUM AND,

. I ALLOYS THEREOF George A; Sandoz, 155 Joliet St. SW., Washington, D.C., and Raymond L. Newbegin, Forestville, Md. (4009 81st Ave., Washington 28, D.C.)

No Drawing. Filed Oct. 30, 1959, Ser. No. 849,990

4 Claims. (Cl. 117-131) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties, thereon or therefor.

invention relates to a method of treating metals,

more particularly refractory metals, to impart oxidation resistance thereto. The refractory metal columbium, possesses the valuable property of high temperature strength which makes it attractive for use in high temperature fields such as in the design of jet engines. Columbium also has possibilities for use in the nuclear field. However, the reactivity of columbium toward oxygen hinders exploitation of its high strength property at high temperatures. The use of columbium in air at temperatures in the neighborhood of 1600 F. and'higher is altogether precluded by a rapid rate of oxidation.

It is a general object of the present invention to advance the use of columbium at high temperatures. It is a particular object to provide a method for treating columbium whereby to provide it with oxidation resistance at high temperatures.

We have found that by the application of the method of our invention to the refractory metal of the group colurnbium and alloys of the same in which columbium constitutes at least 50% by weight of the alloy, including interalloys with vanadium, zirconium, titanium, molybdemun, tantalum and tungsten, oxidation resistance can be provided to themwhich is effective at temperatures up to and in the region of about 1600 F. to 2100" F.

The method of our invention comprises subjecting the columbium coated with zinc directly to the action of heat at a temperature which is slightly below the boiling point of zinc and holding the zinc coated colu-mbium in the heat for a period of time suflicient to form a coating containing zinc and columbium in alloy which is bonded to the surface of the refractory metal.

A simplified flow diagram of the method is as follows. The preliminary step of coating the metal with zinc has been included solely for the purpose of facilitating a ready understanding of the method.

Columbium or the alloys of at least 50% by welght columblum Coating with zinc Heating zinc coated metal to form oxidation resistant coating on the metal 2,991,197 Ratented July 4, 1961 The method of our invention, in contrast to the prior efforts, is capable of providing high temperature oxidation resistant coatings on columbiurn and the aforesaid refractory alloys which have longer'life in service than those heretofore known. The coatings have the unique and valuable property of being self-healing when heated v to temperatures of about 1600?v F. and above. Thus,

erosion and cracking of the coatings which may be sustained in service at a temperature inthe region of about 1600 to 2100. F. can be remedied by the healing of the coatings which will follow from the continued exposure of the coatings at such temperatures. If cracking of the coatings takes place by reason of deformation of the refractory metal substrate at ordinary temperatures, healing of the coatings will be elfected on subsequent heating to 1600 F. and above.

The treatment of the zinc coated refractory metal can be conveniently conducted in a muflie furnace. The temperature of the furnace is kept slightly below the boiling point of zinc (1663 F.) to avoid loss of zinc through volatilization, for which a suitable range of temperat-ures is from about 1550 to 1625 F. An optimum temperature is about 1600 F. The time required to form the desired oxidation resistant coating on the refractory alloys may vary somewhat with the particular alloy.

The columbium or alloy to be treated by the method of our invention canbe provided with a zinc coating in any suitable way. Conveniently, the zinc coating can be applied by hot dipping fro-m molten zinc as in the 'known galvanizing process. The zinc coating may also be applied to the columbiumor alloy by electroplating. For the zinc coating, the colurnbium or alloy should be in clean condition free from grit and grease following good plating practice.

The method of the invention is illustrated by the following specific examples of the practice thereof.

Example 1 A colurnbiuin rod of one-tenth inch diameter was coated with Zinc by hot dipping in molten zinc at 1025 F. The zinc coated rod was allowed to cool to room temperature and then placed in a mufile furnace heated to 1600 F. and, with access of air to the muffle, retained in the furnace at 1600 F. for 15 hours after which it was withdrawn and allowed to cool in the air to room temperature. The formed coating contained zinc and columbium in alloy. Metallographic analysis showed the coating to consist of several distinct layers of a generally darker shade of gray proceeding outwardly from the substrate columbium. The inner zone or layer of the coating next to the columbium was firmly bonded to the latter.

For the purpose of determining the effectiveness of the coating to protect the columbium against oxidation at high temperatures, the coated columbium was subjected to heating at 1800 F. in the presence of air for a period of hours. The specimen was allowed to cool in air at room temperature and then bent through an angle of 90. The colurnbiurn was ductile. The bent specimen was heated at 1800 F. for 2 hours, cooled to room temperature, and bent back through the first bend and through an angle of 180. The columbium was still ductile which is evidence of the absence of oxidation of the columbium. The second heating of the specimen demonstrated the self-healing feature of the coating since the cracking of the coating which occurred in the first bending test was healed by the heat treatment.

Example 2 Stress-rupture specimens of columbium, 0.063 x 0.25" and 1.25" gage length, were coated with zinc by hot dipping in molten zinc and allowed to cool in air to room temperature. The zinc coated specimens were then entered into a mufiie furnace operating at 1600 F. and heat treatment of them continued at 1600 F. in air for a period of 16 hours. The coated specimens from the heat treatment were subjected to stress-rupture tests at 1600 to 1800 F. The stress-rupture tests showed the formed coatings to be efiective to protect the columbium against oxidation at temperatures up to 1600 to 1800 F. while the metal is being deformed under load.

Example 3 Following the treating procedure of Example 1, samples of the alloys CbZOTi, CbIZr, Cb-2Zr, Cb Ti-10Mo and Cb-10Ti40Ta (parts in wt. percent) were zinc coated by hot dipping and heat treated. The coatings formed on these interalloys by the heat treatment were similar to that obtained on the columbium.

Example 4 Repeating the treating procedure of Example 1, a sample of an alloy of Cb40Ti5Cr5Al (parts in wt. percent) was coated with zinc by hot dipping and subsequently heat treated. The coating formed on the columbium alloy was similar to that formed on the columbium.

In the manner of the preceding examples, the method of the invention can be applied to provide oxidation resistant coatings of similar properties to other alloys in which columbium constitutes at least 50% by weight of the alloy, for example, to a columbium base alloy containing 30% by weight vanadium While in the foregoing description of the method of our invention we have made reference to certain specific embodiments, these are to be taken by -way of illustration and not in limitation since the invention may be variously embodied without departing from the spirit or scope of the appended claims.

What is claimed is: 1 1

l. A process of providing oxidation resistance to re- 'fractory metal selected from the group consisting of columbium and alloys of the same in which columbium constitutes at least by weight of the alloy which comprises subjecting the refractory metal coated with zinc directly to the action of heat at a temperature which is slightly below the boiling point of zinc and for a period of time sufiicient to form a coating containing zinc and columbium in alloy and bonded to the surface of the refractory metal.

2. A process as defined in claim 1, wherein the temperature of said heat treatment is about 1600 F.

3. A process of providing oxidation resistance to columbium which comprises subjecting the columbium coated with zinc directly to the action of heat at a temperature which is slightly below the boiling point of zinc and for a period of time suflicien-t to form a coating containing zinc and columbium in alloy and bonded to the surface of the columbium.

4. A process of providing oxidation resistance to co lumbium which comprises subjecting the columbium coated with zinc directly to the action of heat at a temperature of about 1600 F. and for a period of time sufficient to form a coating containing zinc and columbium in alloy and bonded to the surface of the columbium.

References Cited in the file of this patent UNITED STATES PATENTS 1,430,648 Herman Oct. 3, 1922 2,788,289 Deuble Apr. 9, 1957 2,930,106 Wrotnowski Mar. 29, 1960 2,955,958 Brown Oct.- 11, 1960 

1. A PROCESS OF PROVIDING OXIDATION RESISTANCE TO REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM AND ALLOYS OF THE SAME IN WHICH COLUMBIUM CONSTITUTES AT LEAST 50% BY WEIGHT OF THE ALLOY WHICH COMPRISES SUBJECTING THE REFRACTORY METAL COATED WITH ZINC DIRECTLY TO THE ACTION OF HEAT AT A TEMPERATURE WHICH IS SLIGHTLY BELOW THE BOILING POINT OF ZINC AND FOR A PERIOD OF TIME SUFFICIENT TO FORM A COATING CONTAINING ZINC AND COLUMBIUM IN ALLOY AND BONDED TO THE SURFACE OF THE REFRACTORY METAL. 